To get maximum performance from a combustion gun, you want a 1.5 to 1 chamber to barrel ratio. To figure the volume of a cylinder use pi. X radius X radius X length. Soo.. I you have a 4"dia. 14" chamber, you are going to want a 4'4"long 1.7"dia. barrel. If you shoot that little 3 footer at night you will notice a flame about a foot long shooting out the end of your barrel. that's lost performance! you want your burn to end at the exact point that the projectile leaves the barrel. Make a longer one. and you should get another 50 yards or so from potatoes alone. No lie!